In order to vary an output torque according to need, or to control the rotational speed of the prime mover to an efficient speed, the power transmission unit of this kind has been used widely in the prior art. Specifically, a mechanism comprising a plurality of gear pairs is used in general to vary the torque and the rotational speed. In case of using the gear pairs to vary the torque and the rotational speed, power transmission efficiency can be improved in comparison with that in the case of using a belt, a chain or a fluid.
For example, Japanese Patent Laid-Open No. 2002-204504 discloses a power transmitting system composed mainly of a twin-clutch type transmission. The system disclosed in Japanese Patent Laid-Open No. 2002-204504 comprises: two clutch shafts connected selectively with an engine by a clutch; a plurality of gear pairs arranged between the clutch shafts and an output shaft; and a clutch mechanism for connecting the gear pairs selectively with the clutch shaft or with the output shaft. A differential motor generator having a rotor and a stator both of which are rotatable are arranged between those clutch shafts, and connected with each clutch shaft through a bevel gear. That is, the motor generator is arranged perpendicular to the clutch shafts.
On the other hand, Japanese Patent Laid-Open No. 2004-293795 discloses a transmission system, which is adapted to distribute a torque of an engine to two clutch shafts through a differential type motor generator, and to output the torque from the clutch shafts to an output shaft through gear pairs individually set to a predetermined gear ratio.
In addition to above, Japanese Patent Laid-Open No. 2002-155891 discloses a driving device comprising a Ravigneaux type planetary gear mechanism having four rotary elements. An engine is connected with a predetermined rotary element, two of rotary elements serve as output elements, and a motor generator is connected with the remaining rotary element. According to the driving device disclosed in Japanese Patent Laid-Open No. 2002-155891, those rotary elements are individually connected with a gear pair for setting a gear stage, and in case of setting a specific gear stage, the motor generator is halted to use the Ravigneaux type planetary gear mechanism as a speed reducing mechanism.
The systems taught by Japanese Patent Laid-Opens Nos. 2002-204504 and 2004-293795 are adapted to rotate the motor utilizing a difference between a gear ratio of the gear used to drive a vehicle and a gear ratio of the vacant gear. That is, the motor has to be rotated inevitably when the engine is connected directly to drive the vehicle by the power of the engine. Therefore, power loss may be increased as a result of rotating the motor unnecessarily.
On the other hand, according to the device taught by Japanese Patent Laid-Open No. 2002-155891, although such a “drag loss” under the gear stage in which the motor generator is halted can be reduced, the power has to be transmitted through the two gear pairs under the remaining gear stages. Thus, a number of the gear has to be involved in the power transmission and the power loss is therefore increased.